The present invention relates, in general, to an exhaust pipe for an internal combustion machine, and to a method of making such an exhaust pipe.
It is known to use heat insulation to insulate the exhaust system of an internal combustion engine. Typically, the exhaust system has pipelines made of an inner tube and an outer tube, with the air gap between the inner and outer tubes providing a heat insulating effect. This type of heat insulation is however insufficient to comply with increasingly more stringent regulations and to prevent a demanded limited radiation within the engine space. Although the insulation could be improved by incorporating insulating material, this approach leads to even higher temperature differentials between the inner system and the outer system. Although the resulting different thermal expansions of the involved components could conceivably be compensated through provision of compensators, such as sliding fit, there is still a risk, especially when using fibrous insulating material, that individual fibers are blown out through the gap of the sliding fit by the pulsating pressure of the exhaust gas. This is true for any type of sliding fit for the inner system that lacks gastight separation from the flow of exhaust gas. Furthermore, there is a risk that insulant migrates into the sliding fit and may ultimately cause seizing.
Instead of a sliding fit, it has been proposed to use an expansion bellows which is in direct contact with the flow of exhaust gas, or, as disclosed in European Pat. No. EP 0 759 502 B1, to use a combination of a sliding fit and an expansion bellows, whereby the expansion bellows provides the gas tightness of the inner system. The expansion bellows is hereby surrounded by an air gap. When filling the air gap with insulating material, there is a risk that the insulating material interferes with a movement of the expansion bellows and that the insulating effect of the insulant is adversely affected.
It would therefore be desirable and advantageous to provide an improved exhaust pipe which obviates prior art shortcomings and which is capable to compensate substantially thermal length fluctuations in a simple manner, without requiring the need for a sliding fit or other compensating elements. It would also be desirable and advantageous to provide an improved method of making such an exhaust pipe.